Method of conducting in situ combustion



July 21, 1964 DEW ETAL 3,141,502

METHOD OF CONDUCTING IN SITU COMBUSTION Filed Nov. 12, 1959 5Sheets-Sheet 1 (Kl/*Tf) Average 2 Flux, Scf/Hr. Ft.

FIG.

INVENTORS J. N. DEW M. L. MART/N ATTORNEY July 21, 1964 J. N. DEW ETAL3,141,502

METHOD OF CONDUCTING IN SITU COMBUSTION Filed Nov. 12, 1959 5Sheets-Sheet 2 O Q E L6 3 x c o a '0 4 5 m L4 0 LL a: .Q l p 1.2

U m (I '6 1 o l l o INVENTORS J /V. DEW By WLMART/N ATTORNEY July 21,196 J. N. DEW ETAL METHOD OF CONDUCTING IN sn'u COMBUSTION Filed NOV.12, 1959 5 Sheets-Sheet 3 m 6 I kmmu mmom jmz, 20mm wozEbE 4454a ow o owom o on om o o T Sbd mmm O "mmm I n" I o EOE 29582 8 @2502 2253: 8. 6E3?58 July 21, 1964 J. N. DEW ETAL 3,141,502

METHOD OF CONDUCTING IN SITU COMBUSTION Filed Nov. 12, 1959 5Sheets-Sheet 4 A FORMATION CONTAINING LIGHT CRUDE 1 200' L 800' T IHEAVY B CRgJDE LIGHT CRUDE 20% HYDROCARBON PORE VOLUME INJECTED L 220 I780' 41 I T T 90% cm 5A1.

(; l0% GAS sAT. LIGHT CRUDE HEAVY CRUDE AIR OR GAS DRIVE BEFORE IGNITION2o' 1 L 2I5' 765' CLEANED SAND I I COMBUSTED zoNE- D 25%; LIGHT CRUDECOKE ZONE IGNITION STARTED 8I COMBUSTION FRONT ADVANCED 600' 50 350' #4I T I COKED ZONE E CLEANED SAND LIGHT CRUDE COMBUSTION ZONE HEAVY CRUDECOMBUSTION ZONE UGHT F CLEANED SAND I CRUDE com-:0 ZONE INVENTORS F G. 4.1. /v. 05w

BY WLMART/N TTORNEY July 21, 1964 J. N. DEW ETAL METHOD OF CONDUCTING INSITU COMBUSTION Filed NOV. 12, 1959 5 Sheets-Sheet 5 mmDbqmmmEmE.ZOrZsEOu DISTANCE FROM WELL BORE FIG. 54

T MINIMIZED HEAT LOSS DISTANCE FROM WELL BORE FIG. 55

United States Patent 3,141,502 METHQD 0F CONDUCTING IN SITU COMBUSTEGNJohn N. Dew and William L. Martin, Ponca City, Okla.,

assignors to Continental Oil Company, Ponca Qity,

Okla, a corporation of Delaware Filed Nov. 12, 1959, Ser. No. 852,302 6@iaims. (Cl. 16611) This invention relates to a process of starting andpropagating underground combustion and more particularly to a processfor the recovery of oil by in situ combustion from reservoirs containinghigh gravity crude which normally burns by direct air injection withoutleaving sulficient combustion supporting residue for a self sustainedcombustion process in the immediate vicinity of the ignition well bore.

Petroleum is usually found associated with sandstone or porous limestonedeposits situated between impervious layers of shale, or rock and thelike. In most instances, the oil contains lighter gaseous hydrocarbons,such as methane, ethane, propane, etc., which may exist as free gases incontact with the oil or dissolved in the oil itself. The pressure underwhich the oil and associated gases exist is usually proportional to thedepth of the deposit below the surface. When such an oil bearing sand isreached by drilling, oil is produced by flowing to the surface under theexpansive force of the gases at well pressures, whether the gases arefree or dissolved in the liquid oil. Thus, the oil and gas are bothforced into the region of low pressure around the well bottom. Dependingupon the total pressure exerted, and the conditions at the mouth of thewell, the upward movement of the oil and gas may create a flushproduction, for example, in the form of a gusher. During this stage ofproduction, most of the gas associated with the oil escapes, and themotive power bringing the oil to the surface is dissipated.

Pumping is then initiated in order to continue to recover oil. Duringthis stage, gas associated with the oil continues to escape from thecasing head. Subsequently, the flow of oil becomes economicallyunprofitable with respect to further utilization of pumping means.

When the well attains this condition in which the heavier hydrocarbonsobstruct the pores of the sand and no longer flow freely to the wellbottom, the method of repressuring the well system is commonlypracticed. This involves forcing back into selected central wells air orgas which penetrates the sands and finds exit from the adjacent wellscommunicating with the oil reservoir under treatment. The air or gasmechanically forces the crude oil to the venting well bottoms where itis removed by pumping, and the more volatile portions of the residualoil are entrained in the gas or air stream and thus removed from thewell.

In time repressuring becomes no longer expedient, and final resort maybe had to the flooding of the oil field with water to drive additionalamounts of the residual oil contained in these sands into the wells.After flooding has been utilized to the point where it is no longerprofitable, the flooded field becomes non-productive and must beabandoned.

It is well known that in the fields subjected to the foregoingtreatment, nearly half of the oil known to be initially present is stillleft as residual oil in the sands. At the present time, the problem ofrecovering this vast amount of residual oil has become an urgent one,particularly due to the increasing demand for petroleum and petroleumproducts, and the rapidly diminishing number of discoveries of new oilfields.

It has heretofore been recognized that oil may be recovered by applyingheat to oil-containing sands in their native position. By thus heatingthe oil-containing sands,

3,3415% Patented July 21, 1964 the heavier hydrocarbons clogging thepores of the sand are rendered less viscous, and the flow thereofthrough the sand is facilitated. In addition, the more volatilehydrocarbons are distilled from the sand to the venting well casing.Various methods have been proposed for effecting a heating of the oilsands. For example, direct combustion of a portion of the residual oilutilizing air under pressure, or air and a combustible gas to initiateand support combustion of the oil has been proposed. It has also beenproposed to pass heated products of combustion in gaseous form throughthe oil sands in order to effect a heating thereof, whereby theviscosity of the residual oil is reduced and the oil becomes moremobile. However, these prior methods have certain inherent disadvantageswhich are obviated by the present invention.

In the subterranean combustion process, the oxidizing gas, which may beair, oxygen, oxygen-enriched air, air admived with inert gas to reducethe proportion of oxygen, oxygen admixed with inert gas, or any othersuitable oxidizing gas or mixture which will support combustion withinthe subterranean reservoir, is passed, as by pumping, through an inputwell to the reservoir in which the combustion process is to be effectedand combustion within the reservoir is initiated by suitable means. Theflow of oxidizing gas to the reservoir is continued and the combustiongases, oil, and the distillation and viscosity breaking products migratein front of the combustion zone to an output well or wells leading fromthe reservoir, from which output well or wells these fluids are removedand thereafter treated for recovery of the desired valuableconstituents. The heated fluids migrating in front of the combustionzone strip the oil-bearing sand of the greater portion of the oilleaving behind within the sand a carbonaceous hydrocarbon deposit. Thecarbonaceous deposit essentially is the fuel consumed in the process andcombustion of the carbonaceous deposit continues until the deposit hasbeen substantially entirely consumed. The carbonaceous depositpreferentially burns to form carbon dioxide irrespective of restrictionin the supply of oxidizing gas and thereby provides a maximum amount ofenergy per unit amount of carbonaceous deposit consumed. However, wherethe amount of carbonaceous deposit is in excess of about 2 percent byweight of the sand, combustion of the carbonaceous deposit provides morethan sufficient thermal energy for achieving efficient recovery ofpetroleum oil by the combustion process,

and the additional amount of oxidizing gas consumed by the carbonaceousdeposit constitutes an economic waste not only with respect to theexcessive amounts of oxidizing gas which must be pumped to the reservoirbut also with respect to the excess pump capacity required.

One such method comprises establishing a combustion zone around aproduction well by conventional methods so as to provide a combustionzone and a heat reservoir of sufficient extent and temperature to permitcutting oif the direct flow of air through the production well andinjecting air into the formation through one or more spacedapart wellsfrom the production well so as to cause the air to flow to thecombustion zone at the production well and support combustion therein sothat the combustion front is advanced countercurrently to the flow ofair toward the injection well or wells. This technique is designatedinverse air injection in situ combustion as opposed to direct airinjection through the well or bore hole around which combustion isinitiated.

Another recent development in recovery of oil by in situ combustioncomprises continuing the injection of air through one or more injectionwells after the combustion front has been advanced, by inverse airinjection, to the injection well or Wells so as to reverse the movementof the combustion front and drive the same back through the formation tothe production well around which combustion was originally initiated. Inthis technique, designated thermal echo, the returning combustion frontfeeds on the residual carbon deposited in the formation during theinverse air injection phase of the process.

It has been found that in many oil-bearing formations the crude is ofsuch high API gravity (and low carbon residue) that the hot gases from acombustion front initiated around an injection Well drive thehydrocarbon materials substantially completely away from the area infront of the combustion front thereby leaving insufficient fuel tosustain combustion and the fire goes out. This renders it impossible toinitiate combustion and build up a sufficient combustion zone and heatreservoir to permit reversing the direction of the flow of air to thecombustion zone in order to establish inverse air injection to supportand drive the combustion zone toward the surrounding injection wells andaway from the well in which the combustion is originally initiated.

J. C. Trantham and H. 0. Dixon in US. Patent 2,889,881 disclose a backburn process for the recovery of oil which comprises depositing a heavyhydrocarbon oil, such as heavy crude oil, in the formation surrounding aproduction well or bore hole and initiating combustion of the depositedheavy oil so as to burn a sufficient amount of the oil to establish acombustion zone which contains enough heat to permit cutting off thedirect injection of air or other combustion-supporting gas and feedingair to the combustion zone from one or more spaced-apart wells or boreholes through the formation whereby the combustion zone is advancedtoward the injection well and combustion products and producedhydrocarbons are driven to the production well from which they arerecovered by conventional means. The heavy oil is injected into theformation through a well or bore at a selected location so as topenetrate the formation for several feet (at least 3 or 4 and preferablyto feet) surrounding the bore hole. The deposited oil is then ignited byany suitable means such as by the use of a squib, an electric heater, agas heater, or any other heating device supplemented by injection of anoxygen-containing gas (preferably air) to support the combustion of theheavy oil. It is also feasible to pump hot air at combustion supportingtemperatures down the well or bore hole until the temperature of thedeposited oil and formation which it occupies are brought to atemperature sufiicient to initiate combustion which is in the range ofapproximately 500 to 700 F.

Burning of the oil in the formation surrounding the bore hole leaves acarbon residue which is sufficiently hot to support combustion when thedirection of the air is reversed and fed thereto from surroundinginjection wells. The quantity of deposited oil burned in the formationmust be sufiicient to build up a reservoir of heat which holds thetemperature sufficiently high to support combustion when the inverse airreaches the combustion area. It is essential to burn only a portion ofthe deposited heavy oil so that a reserve is left to preventvaporization of all of the original oil in the formation directlyoutside of the deposited oil and to provide a continuous bed of fuelfrom the heat reservoir or combustion zone to the injection wells. Thearrival of the inverse air from. the surrounding injection wells thenrevives the combustion zone. After inverse air injection and attendantcombustion is established, the remaining heavy oil is burned and, as theinverse air injection is continued, the combustion front is propagatedradially outwardly and laterally past the limit of the heavy oilsaturation and the burning is continued in the light oil reservoir so asto advance the combustion front to the injection wells.

It is an object of our invention to provide a process for initiating insitu combustion and recovery of oil in an oil reservoir containing crudeof high API gravity by means of a forward drive process. Another objectof our invention is to provide a method of oil recovery by in situcombustion from an oil bearing formation which is incapable ofsupporting in situ combustion during ignition by direct air injection.Other objects and advantages of the invention will become apparent asthe invention is hereinafter disclosed.

Other objects and advantages of the invention will be evident from thefollowing detailed description when read in conjunction with theaccompanying drawings which illustrate our invention wherein:

FIGURE 1 is a graphic representation of the relationship of the ratio ofeffective formation gas permeability in millidarcies to the product ofcentipoise gas viscosity and formation temperature expressed in Rankineto the ratio of the distance of a combustion front from an injectionwell to the distance between the injection well to a production well.

FIGURE 2 depicts the relationship between fuel requirements forsupporting combustion and the rate of advance of a combustion front.

FIGURE 3 shows the relationship between fuel residue in a combusted areanecessary to support combustion and the radial distance from aninjection well.

FIGURE 4A is a schematic representation of a linear system of porousmedia containing a crude oil not capable of supporting self-sustained insitu combustion.

FIGURE 4-B is a schematic representation of the systerrli of 4-A afterinjection of a slug of bituminous materra FIGURE 4-C depicts thedisplacement of bitumen into the system of 4-B by air or gas injection.

FIGURE 4D depicts the movement of a combustion front in the system ofFIGURE 4-C after ignition of the bituminous slug.

FIGURE 4-E shows the condition of the system shown in FIGURE 4-D afterfurther movement of the bituminous slug and combustion front.

FIGURE 4-F shows the condition of the system of FIGURE 4-E when thecombustion front nears the outlet face of the porous medium.

FIGURE 5A shows formation temperature distributron characteristics in arecovery process using well bore heaters.

FIGURE 5B shows formation temperature distributron. characteristics in arecovery process using a moving in situ combustion front.

The foregoing objects and advantages are attained by a process which maybe described briefly as follows: A low gravity crude oil which is richin high boiling fractions and carbon residue is injected into anunderground formatron containing hydrocarbons not capable of supportingself sustained combustion. The injected crude oil is then dispersed intothe formation by means of gaseous pressure after which the injectedcrude oil is ignited. The combustion front is propagated through theformation by the continued injection of a combustion supporting gas intothe formation.

Secondary recovery of oil by in situ combustion is a widely discussedmethod that is finding increasing applicat on. This method is basedfundamentally on propagating a combustion front through a permeable oilsand from air injection wells toward oil recovery wells. Heat generatedby burning a portion of the oil in place provides a mechanism forrecovery of the remaining oil. The front moving outwardly from theinjection wells actually consists of several contiguous zones:

(1) A completely combusted Zone (2) The burning zone (3) A coke zone (4)A vaporization-condensation zone (5 A zone flowing three fluid phases(oil, gas, and water It is the coke zone that keeps the combustionprocess going. Coke is generated as heat from the burning zone istransferred to the oil bank in the vaporization zone ahead of the front.The amount of coke that the oil originally in place can deposit on thesand is a critical factor in propagating the combustion front throughthe sand.

We have found that a certain amount of coke must be continuouslydeposited if the combustion process is to be self-sustaining. The heatavailable by the combustion of this coke must be enough to raise thetemperature of the associated rock solids to combustion levels andreplace heat losses to the surroundings. This requirement decreases withan increase in original reservoir temperature; therefore, all otherfactors being equal, the coke laydownrequirements will decrease with thedepth of the reservoir. If heat losses are high, coke requirements tomaintain self-sustaining combustion temperatures will also be high. Ifthe amount of coke deposited per cubic foot of sand swept drops below acritical value fixed by rock heat capacity and heat losses, combustionwill eventually cease. Fortunately, the minimum amount of coke requiredto maintain combustion at low rates of frontal advance declines as thecombustion front moves away from the well bore, due primarily to preheatof the formation ahead of the combustion front. For instance, for areservoir of 15% porosity at rates of advance in the order of 0.2 footper day, about 4 pounds of coke per cubic foot of rock may be requiredto maintain combustion at a distance of 5 feet from the well bore; butat a distance of 30 feet, only 2.5 pounds of coke per cubic foot of sandmay be required. Furthermore, the coke requirement decreases as the rateof advance of the combustion front increases. These differences in cokerequirements can be attributed to reduced heat losses from thecombustion zone as stabilization of the temperature profile of theadvancing combustion front is approached.

There are many reservoirs that contain crude oils which are incapable ofsupplying sufiicient coke to maintain in situ combustion near theinjection well bores, while some oils are incapable of supplyingsufl'icient coke to maintain combustion in any portion of the reservoir.Frequently, these types of hydrocarbons are found in low porosity andlow permeability host rocks. These rock properties further jeopardizethe prospects for a selfsustaining combustion front, because we havefound that the minimum coke requirement increases as the porositydecreases. The low permeability prevents the use of high air fluxes withcorrespondingly high rates of advance and reduced coke requirements.

We have discovered that a combustion wave can be initiated andpropagated through a given body of porous sandstone with or withoutliquid saturations by means of the following steps:

(1) Injecting an appropriate fraction of a pore volume .of a hydrocarbonoil, such as a low gravity crude oil, out back asphalt, or residualhydrocarbon material (preferably a bituminous material rich in highboiling fractions 7 and carbon residue) into the sand body.

(2) Further displacing the hydrocarbon into the sand by means of airpressure and air sweep.

(3) Raising the temperature of the air inlet sand face to the ignitionpoint of the hydrocarbon and/ or its residuum or coke.

(4) Propagating the combustion front through the sand by continued airinjection.

We have further discovered that the distance through which selfsustained combustion can be maintained in porous media is dependent on,and can be controlled by, the amount of hydrocarbons injected.

The heat loss from a moving combustion front varies inversely with thevelocity of its advance through the porous media. The rate of advancevaries directly with air flux (air rate per unit area) at the front.

The rate at which air or gas will flow between wells in a permeablereservoir may be calculated by means of an appropriate equation based onDarcys law. Equation 1 describes the flow of gas from a centralinjection r 6 well to four production Wells drilled in an isolated S-spopattern.

During an in situ combustion operation, the combustion front progressesoutwardly from the central injector toward the production wells. Thereservoir rock behind the combustion zone is completely devoid of theoriginal fluids and is at an elevated temperature. Thus, the effectivepermeability of the reservoir to the flow of injected air or gas ishigher behind the front than ahead of it, and the viscosity and volumeof gas is also higher behind the front. The ratio of permeabilities mayvary about 3.3 and 10.

An average value for the ratio (k /nT which appears in Equation 1 may becalculated by means of Equation 2.

Values of the ratio (k T are shown by curves 1, 2,, and 3 on FIG. 1 as afunction of the location of the combustion front and the size of wellbores for Kl/Kg equal to 10. As shown in FIG. 5 of the paper, ProcessVariables of In Situ Combustion, by Martin, et al., published inFebruary 1958, Journal of Petroleum Technology, the rate of advance of acombustion front in an oil reservoir is directly related to the air fluxat the front.

With high combustion efficiencies this may be expressed by the followingequation:

M( ROA 3 where u=air flux-s.c.f./ hr. -ft. A air required/cu. ft. sandcleaned but t,(1000 t, s.c.f. 4)

Definition 0 Symbols The air flux may be estimated at any desiredlOCatlOIl between wells by means of Equation 4 above. Let us assume thecharacteristics of one of the well-known, high-gravity crude oilreservoirs. Thus, for each foot of not pay in the Bradford Pool inPennsylvania with K =l0 md. and K rl md., a maximum injection pressureof 1000 psi. and 2 /2 acre 5-spot spacing, the maximum attainable airflux according to Equation 4 would vary with location, as shown by curve4, FIG. 1. The corresponding rates of advance calculated by Equation 3with A ==420 s.c.f./cu. ft. are shown on curve 1, FIG. 2. As can beseen, the maximum attainable rate of advance decreases rapidly as thefront moves away from the injection well if injection pressures andrates are limited, as they commonly are, by formation depth orcompressor rating.

As stated previously, the heat loss from an advancing a front variesinversely with velocity. The fuel required to replace these heat lossesmay be calculated with equations based on the laws of unsteady-stateheat transfer. A method for using an analogue computer to solve theequations for fuel requirements has been described in a paper by Vogeland Krueger in the AIME Journal of Petroleum Technology, vol. 7, pp.208-209, December 1955, entitled An Analogue Computer for Studying HeatTransfer During a Thermal Recovery Process.

By methods described by Vogel and Krueger, it can be shown that the fuelrequired to replace heat losses varies with location of the combustionfront and its velocity of motion or rate of advance. Typical data areshown on FIG. 3.

If the data of FIG. 3 are used in conjunction with the rate of advanceand location data of FIG. 2, an estimate of the fuel required per cubicfoot of Bradford sand may be deduced. These data are shown as curve 2 onFIG. 2. The dotted line on this curve indicates the effect of decreasingvertical sweep efficiency (channeling) which may occur as the frontmoves out from the injection well.

The following conclusions may be drawn, based on the data of FIG. 2:

(1) The maximum attainable rate of advance decreases rapidly withdistance as the front moves away from an injection well bore.

(2) When low formation permeability and/ or reservoir depth limitsinjection rates and pressures, fuel requirements are maximized in thevicinity of the injection well bore.

Our invention consists of supplying the required amounts of fuel at thecombustion front during its movement through this critical region. We dothis by injecting an appropriate amount of a bituminous material rich inhigh boiling fractions and carbon residue. This material is then furtherdisplaced into the formation by air or gas injection and ignited at thesand face by methods known in the art. The combustion front is thenpropagated away from the well bore at a velocity selected so as toprovide for stabilization of the advancing temperature profile of thecombustion front at an appropriate location away from the injection wellbore. At this location, the crude oil originally in place can supply therequired amount of coke deposition. The minimum fuel requirements forsome low porosity, low permeability reservoirs may be in excess of thatprovided by the crude oils contained therein. Thus an extension of ourinvention consists of injecting a selected sized slug of high boiling,carbonaceous bitumen, so as to provide sufficient fuel to propagate thecombustion front over the entire distance between wells. This scheme maybe particularly attractive in those areas where heavy, black crudes area glut on the market (or where heavy crude is easily imported) and highgravity crudes bring premium prices. The data cited above indicate thatsuch a scheme would be required to propagate a combustion front betweenwells in pools such as the Bradford sand in Pennsylvania and the Poncasand in the field near Ponca City, Oklahoma. Our data for the lattercrude oil show that it does not provide sufficient fuel forself-sustained combustion even in high porosity, unconsolidatedsandpacks.

To explain further the application of our invention, reference may bemade to FIG. 4. FIG. 4-A is a schematic representation of a linearsystem of porous media containing a crude oil not capable of supportingself-sustained in situ combustion. FIG. 4-B depicts the injection of aslug of bituminous material which has been sized to provide sufficientfuel to propagate a combustion front through the media. FIG. 4-C depictsthe displacement of the bitumen into the media by air or gas injection.The

resulting low gas saturation provides gas permeability through thebitumen and causes an expansion of the size of the slug. FIG. 4-Ddepicts the movement of the front and the slug into the porous mediaafter ignition by continued air injection, As the front progressesthrough the media, the bitumen is consumed for fuel and the size of theslug decreases. However, the movement of the front drives the viscousbitumen slug through the media, thus effectively displacing the lessviscous and more valuable crude oil originally present in the porespaces. FIG. 4-E shows how the size of the ,slug has continued todecrease as the front progresses, and FIG. 4-F shows that the slug isessentially consumed as the front nears the outlet face of the porousmedia.

In practice in the field, the slug size used must provide for theeffects of the expanding radius of the burning front as it moves awayfrom the injection well. This is best done on the basis of the volume ofrock to be swept and the corresponding fuel requirement. Our dataindicate that almost complete recovery of the lighter crude oil may beaccomplished by only propagating the combustion front part of thedistance between wells. The smallest slug is that required to provide acritical temperature increase which may be desirable to effect completerecovery of fluids from condensate reservoirs and crude oil reservoirswhich can support combustion at distances away from the injection well.

The procedure depicted in FIG. 4 has advantages over processes dependenton well bore heating. FIG. 5 shows the formation temperaturedistribution characteristic of the two procedures. FIG. 5-A shows thatprocesses using well bore heating methods, such as downhole burners,steam generators, heaters, etc., require that the entire formationswept, from injection well outwardly, be held at a high temperature.This results in high heat losses to overand under-burden. FIG. 5B, incontrast, illustrates the relatively small proportion of the totalreservoir which is required to be at a high temperature for the processwhich uses a moving in situ combustion wave for oil recovery.Furthermore, a combustion front can be moved more rapidly through areservoir than a heat wave can be moved by the injection of heatedfluids, further reducing the heat losses to the surroundings. Thedecreased heat losses accomplished by our invention, i.e. injectingbituminous material into the porous media and buning in situ, isreflected in lower fuel requirements in terms of surface barrels of fuelrequired to provide the recovery desired from the reservoir. This methodof operation produces 2 to 3 barrels of oil originally in place for eachbarrel of bitumen injected. Based on price differential, the cost offuel would be in the order of 1 dollar for each 4 to 6 dollars returned.

As to the amount of injected oil required for practicing the inventionthat can be calculated as follows: The first step is to determine theamount of coke-laydown per cubic foot of porous rock that is necessaryto maintain the combustion process. This can be done by referring toFIG- URE 2 where coke required is plotted on the left-hand ordinateversus the fraction of the distance between in the injection andrecovery wells that the flame front has covered (r/d). Near the wellbore the amount of coke necessary will be seen to change rapidly, but ata point farther from the well the amount necessary levels off to about1.2 pounds of coke per cubic foot of rock.

After the amount of coke necessary has been determined the proper crudemust be selected for the particular reservoir. The proper crude will beone that supplies the amount of coke necessary as determined in thepreceding paragraph. A prospective crude should be tested in alaboratory in situ combustion apparatus. As a result of considerableexperimental work we have found that crudes of the following APIgravities provided the amounts of coke indicated.

Pounds coke per API: foot of rock The amount of oil to be injected intothe reservoir can now be calculated as follows:

Volume of oil (in barrels)=(0.125)(V)(l-) where V=volume of thereservoir in ft. to be swept up to the point where combustion willbecome self sustaining. =the pore fraction of the reservoir.

While the instant invention comprises the several steps and the relationof one or more of such steps to each of the other enumerated steps, itis to be clearly understood that various changes in the method ofprocedure may be resorted to without departing from the spirit of theinvention, and further that the theories set forth, although believed tobe accurate, are not to be considered as the sole basis of theoperativeness of this invention, but that this method does operatesuccessfully and effectively Whether or not upon the principlesdescribed herein, this invention to be limited only by the appendedclaims. Minor changes to make a preferred adptation to any particularsituation may be readily made by one skilled in the art, as trial mayindicate.

We claim:

1. In a process for recovering high API gravity hydrocarbons through arecovery well from an underground formation wherein said high APIgravity hydrocarbons are characterized upon combustion by insufficientcoke laydown to provide self-sustained combustion in the vicinity of aninjection well, the method which comprises the steps of injecting intosaid formation through said injection well a quantity of hydrocarbon oilsufficient to support combustion to a point in the formation wherein thehigh API gravity hydrocarbons are capable of providing self-sustainedcombustion, injecting gas through said injection well into saidformation to disperse said hydrocarbon oil into said formation towardsaid recovery Well, igniting said hydrocarbon oil, injecting acombustion supporting gas into said formation through said injectionwell to propagate the combustion front through said formation towardsaid recovery well and recovering hydrocarbon by way of said recoverywell.

2. The process of claim 1 in which the hydrocarbon oil is a low gravitycrude oil.

3. The process of claim 1 wherein said combustion supporting gas is air.

4. The process of claim 1 wherein the combustion supporting gas isagascontaining free oxygen.

5. The process of claim 1 wherein the injected crude oil has an APIgravity varying from 10.9 to 41.7.

6. The process defined in claim 1 wherein the quantity of hydrocarbonoil is sufficient to support combustion only to the point in theformation whereat the high gravity hydrocarbons are capable of providingself-sustained combustion.

References Cited in the file of this patent UNITED STATES PATENTS2,889,881 Trantham et al June 9, 1959

1. IN A PROCESS FOR RECOVERING HIGH API GRAVITY HYDROCARBONS THROUGH ARECOVERY WELL FROM AN UNDERGROUND FORMATION WHEREIN SAID HIGH APIGRAVITY HYDROCARBONS ARE CHARACTERIZED UPON COMBUSTION BY INSUFFICIENTCOKE LAYDOWN TO PROVIDE SELF-SUSTAINED COMBUSTION IN THE VICINITY OF ANINJECTION WELL, THE METHOD WHICH COMPRISES THE STEPS OF INJECTING INTOSAID FORMATION THROUGH SAID INJECTION WELL A QUANTITY OF HYDROCARBON OILSUFFICIENT TO SUPPORT COMBUSTION TO A POINT IN THE FORMATION WHEREIN THEHIGH API GRAVITY HYDROCARBONS ARE CAPABLE OF PROVIDING SELF-SUSTAINEDCOMBUSTION, INJECTING GAS THROUGH SAID INJECTION WELL INTO SAIDFORMATION TO DISPERSE SAID HYDROCARBON OIL INTO SAID FORMATION TOWARDSAID RECOVERY WELL, IGNITING SAID HYDROCARBON OIL, INJECTING ACOMBUSTION SUPPORTING GAS INTO SAID FORMATION THROUGH SAID INJECTIONWELL TO PROPAGATE THE COMBUSTION FRONT THROUGH SAID FORMATION TOWARDSAID RECOVERY WELL AND RECOVERING HYDROCARBON BY WAY OF SAID RECOVERYWELL.